[jbattle]

It has to do with the class of star. This star is so cool that the only way for the planet to be warm enough to support life is for the planet to be really close to the star. I don't really understand tidal locking but I think it happens vastly faster at closer orbits - because there is a stronger gravitational gradiant closer to the parent body.

[XorNot]

Tidal lock is caused by differential gravitational attraction. The half of the Earth currently facing the sun experiences more force pulling it towards the sun then the part facing away - which manifests as the spin of the planet having to do work against gravity to keep the spin going.

As a planet gets closer and closer to it's star, this gets more and more pronounced and all the various deformations and instabilities begin to favor braking the planet rather then it continuing to spin.